On my 1980 300D,617 912 engine I checked mine a while back.Unplug the connector from the relay and you should see the numbers 1 3 5 7 across the top and 2468 at the bottom.#1=glowplug(gp)#2 ,#2=gp#1 ,#3=gp#3 ,#4=not used ,#5=gp#4 ,#6=temp sensor ,#7=gp#5 ,#8=unplugged connector not sure what it's for at engine. Hope this helps
